Quantum key distribution (QKD) refers to communication methods that use quantum mechanical principles to guarantee secure communication between two parties. In QKD, a (random) secret key is shared between the two parties, where the key is known only by the two parties to the communication. The key is used to encrypt and decrypt messages. Security of communication between the two parties is assured as a result of the quantum uncertainty principle. If an eavesdropper on a communications channel measures data being transmitted, anomalies are introduced in the data that are then passed down the communications channel and received by a receiver that is party to the secure communication. The receiver can detect the eavesdropper by observing the presence of these anomalies, and can either cease the communication or discard compromised bit values of the shared key.
Discrete variable QKD (DV-QKD) systems modulate and analyze properties of single photons in optical signals to encode and decode data for QKD. DV-QKD systems can encode data on polarization states of single photons, where a polarization state of a photon can represent a logic “1” or a logic “0”. A DV-QKD protocol, BB84, defines a method for performing DV-QKD in which two measurement basis sets are used by a transmitter to transmit information, wherein each basis set defines two different photon polarization states corresponding to the two logic states, for a total of four possible polarization values. A receiver, not knowing which basis set a photon was transmitted in, measures the polarization state of the photon in a randomly-chosen basis. The transmitter and the receiver then compare chosen bases for each measurement to securely determine which of a plurality of communicated bits will make up a shared encryption key.
Continuous variable QKD (CV-QKD) systems modulate and analyze phase and amplitude of continuous low-intensity optical signals in order to encode and decode data for QKD. CV-QKD systems, like DV-QKD systems, typically use communications protocols that call for sending data on two different measurement bases and comparing the measurement bases used during reception in order to securely exchange a shared encryption key. Conventional CV-QKD systems require a local oscillator signal to be separated from a data signal at a transmitter prior to modulating the data signal to encode some data. The local oscillator and the data signal are then recombined and transmitted on a communications channel to provide a way for a transmitter and a receiver to measure signal features from a common reference. QKD systems have conventionally relied on bulk fiber-optic components such as Faraday mirrors and long fiber-optic delay lines in order to maintain phase coherence between the local oscillator and the data signal.